1. Field of the Invention
The present invention relates to a method and apparatus for controlling the air-fuel ratio of a gas engine by controlling the air-fuel ratio on an exhaust purifying apparatus such as a three way catalyst associated with the gas engine.
2. Background Discussion
A gas engine system suitable for use in the present invention is shown in FIG. 10. As shown in FIG. 10, on an intake line 1 of gas engine E, there is positioned a mixer 4 for mixing fuel (such as town gas 13A) supplied from a fuel line 2 with air supplied from an air intake 3, a throttle valve 5, and a pressure sensor 6 detecting intake pressure.
Also, on by-pass line 7, which connects the fuel line 2 to a line between the mixer 4 on the intake line 1 and the throttle vale 5, there is positioned by-pass valve 18 which is in the form of an air-fuel ratio control valve.
On the other hand, on exhaust line 9, three way catalyst 10 is interposed. On an up-stream side of three way catalyst 10, there is positioned a main oxygen sensor 11 and a front (up-stream side) thermometric sensor 13. Also, on the down-stream side of three way catalyst 10, there is provided sub oxygen sensor 12 and a rear (down-stream side) thermometric sensor 14. (The phrase "up-stream" means a side near the intake 3. On the other hand, the phrase "down-stream" means a side away from said intake 3.)
The pressure sensor 6, both of the oxygen sensors, and the air-fuel ratio control value 18 are connected to a control unit, respectively, which unit is represented by reference numeral 15. The control unit 15 sets up a basic by-pass quantity in response to a signal transmitted from pressure sensor 6, sets up a control constant based on a main feed-back from the main oxygen sensor 11, determines a compensating by-pass quantity by compensating the control based on a sub feed-back from the sub oxygen sensor, compensates the basic by-pass quantity due to the compensating by-pass quantity, and sets up the final by-pass quantity so that said unit controls the drive current driving the air-fuel ratio control value.
It is well-known that, in order to make the three way catalyst effectively function in the above-mentioned control method, an air-fuel ratio between air and fuel gas should be adjusted within an extremely narrow range, the so-called window (Approximately 0.995/0.005, in air-fuel ratio). In the above-mentioned control method, when a mixed state of air and fuel gas through mixer 4 is kept a little leaner (surplus air state), a shortage of fuel gas is supplied by air-fuel ratio control valve 18 (via the by-pass line 7). In this situation, the supply of fuel gas is carried out as follows: